Could you expand on "hours candidate can really-work"?
I, personally, rarely can do really-work more than 4 hours a day -- only if task is very interesting and challenging. Then I can do it for 12 hours a day straight, but I stop myself if I work more than 6 hours, because I know I would burn out, happened a few times.
4 hours a day of really-work sounds not a lot, but my performance is pretty good comparing to my teammates. Never heard managers complain on my performance. The other 4 hours I spend on meetings/lunch/hacker news/socializing etc. So I would look for a candidate who can do it for at least 4 hours a day.
I heard rumors of people who can really-work for 8 hours a day (like John Carmack). Those people are called "100x engineers", I really want to work with some of them one day.
But I can produce just as much as some other developers, so I really see no incentive to do more work unless it's clear I will get more recognition for it, rather than just more work to do.